It is almost a trite observation to note that the use of a broad spectrum of drugs to treat human and other mammalian disease is routine medical and veterinary practice. Therapeutically active agents, however, often suffer from a number of shortcomings which limit and complicate their use. A particular problem is that, after administration to the patient, a drug may be so rapidly cleared form the body by metabolic or other pathways or otherwise biologically inactivated so that only a relatively small percentage of the drug administered actually has a therapeutic effect. To compensate for this problem, it is common practice to increase the dosage of the drug and/or to prolong its period of administration and/or to shorten the interval between doses so that the therapeutically effective concentration of the drug in the patient is maintained for a period sufficient to achieve the desired result.
These procedures are useful but have their own limitations. Increasing the dosage may be limited, for example, in the case of intramuscular administration, by the bolus which can be tolerated. Many drugs have toxic side effects which may limit the dosage duration or interval which can be safely used. In some cases, promising drugs cannot be used because side reactions are so severe that an effective therapeutic dose cannot be safely administered. The need to administer multiple small doses of a drug or to use continuous infusion techniques increases the cost of treatment and the burden of hospital personnel, and, of course, adds to the patient's discomfort.
Accordingly, there exists a need for means by which the therapeutically active concentration of a drug, after administration, is maintained for a longer time.